The present invention relates in general to a coil system for producing a gradient magnetic field used in magnetic resonance imaging and spectroscopy, and more specifically to a coil system for producing a magnetic field having a linear gradient in the interior of the coil system and having a substantially zero value exterior of the coil system.
Magnetic resonance (MR) imaging systems are currently employed in forming tomographic images of the internal human anatomy. In such systems, a patient is placed in a static magnetic field and is subjected to radio-frequency electromagnetic pulses. The magnetic resonance of the atomic nuclei of the patient are detected with an antenna to provide information from which an image of that portion of the patient containing these nuclei may be formed. The magnetic field possesses linear gradients in each of the three spatial dimensions so that the position of resonating nuclei can be determined. These same phenomena are employed in magnetic resonance spectroscopy for analyzing properties of nuclei.
Typically, a main magnet establishes a uniform magnetic field of high strength and the linear gradients are superimposed on that magnetic field by gradient coils located within the main magnet interior. The main magnet may be comprised of a superconducting coil, a resistive coil, or a set of permanent magnets. A gradient coil may be comprised of windings etched on a flexible printed circuit board which is curved to form a cylinder or may be comprised of conductors held by a frame in their proper locations.
Bringing the inner bore of the main magnet close to the imaging volume, and thus close to the gradient coils, enhances the efficiency of the magnet in terms of amount of material, cost and, except for permanent magnets, power consumption. However, the close proximity of the magnet to the gradient coils leads to undesirable interactions between them.
For instance, where the main magnet includes electrically conductive material (e.g. neodymium permanent magnets or the cryostat of a superconducting magnet), the changing magnetic fields of the gradient coils induce currents in the conductive material. These currents distort the magnetic field in the imaging volume in both time and space. Furthermore, when permanent magnets are used, the eddy currents dissipate power which appears as heat, distorting the magnetic field even more. The solution of placing a conducting shield between the gradient coils and the magnets merely shifts the eddy currents to the shield and so the distortions of the gradient fields persist.
Magnetic interaction between the main magnet and the gradient coils also results in attractive and repulsive forces which create annoying audible sound. Structural supports having high strength and rigidity have been employed in attempts to reduce audible sound from the gradient coils, but with limited success.
Accordingly, it is a principal object of the present invention to reduce interaction between the main magnet and the gradient coils of an MR imaging system.
It is another object of the present invention to provide a coil system for producing a magnetic field having a substantially linear gradient in the interior of the coil system and having a substantially zero value exterior of the coil system.
It is a further object of the invention to practically eliminate currents in the main magnet induced by the magnetic fields of the gradient coils.
It is yet another object of the invention to reduce audible sound from the gradient coils of a nuclear magnetic resonance imaging system.